1. Field of Invention
The present invention relates to a scanning apparatus, more particularly to a scanning apparatus having a plurality of point light sources to emit a scanning light.
2. Related Art
A scanning apparatus includes an image sensor, a light emitting element, and optical elements. The optical elements are provided for reflecting, refracting, or focusing light, so as to change a light path of the light and focus the light in a specific area. The light emitting element is provided for emitting a scanning light onto a document. The scanning light is reflected by the document to form an image light that is then received by the image sensor and converted into image data.
As the image sensor receives the image light and then convert it into the image data, the spectrum and intensity of the light emitted by the light emitting element are quite important. The spectrum of the light has to fall within a predetermined range, such that the reflected image light can properly reproduce the color of the image of a scanned document, and the image sensor can capture the correct image. The intensity of the reflected image light has to be sufficient enough for the image sensor to correctly capture image. In addition, the scanning light intensity has to be uniformly distributed to avoid the brightness non-uniformity on the document, so as to the reflected image light can be uniformly for the image sensor to correctly capture the image. As the image sensor is elongated and the image sensor scans the document through a slit to capture the entire image of the document, the ideal light emitting element is a linear light source. Conventionally, a cold cathode fluorescent lamp (CCFL) is usually used as the light emitting element to serve as a linear light source with uniformly distributed light intensity, and to provide an ideal white light. However, the CCFL has some disadvantages. First, the CCFL has a high operating voltage and a higher discharge voltage to activate the CCFL, so a high voltage driving circuit is required to provide a high voltage to activate and drive the CCFL, thus causing the increase of the cost of the scanning apparatus. Second, it is required to wait a period of time for the intensity of the emitted light becoming stable after the CCFL is activated. That is to say, the scanning apparatus using the CCFL after being started is required to wait a period of time for warming up, and then performs operation. Third, the intensity of the light emitted by the CCFL is greatly influenced by temperature. If the temperature of the CCFL increases after the scanning apparatus performs uninterrupted operation, the intensity of the emitted light is also changed with the change of the temperature, and even the spectrum is shifted, such that the emitted light is not the ideal white light anymore, and the image scanning quality under the uninterrupted operation of the scanning apparatus is inconsistent.
In addition to the CCFLs, LEDs also emit the ideal white light, so that the LEDs can be applied in the scanning apparatus to serve as the light emitting element for providing the scanning light. The LEDs are not limited to white LEDs capable of emitting the ideal white light. For example, in R.O.C Taiwan patent TW518877, the scanning apparatus does not apply white LED, but to use a monochrome image sensor to capture the image of different colors in a sequence of red, green, and blue lights, and then combine them into a color image, thereby solving the problem of high costs of the white LED and the color image sensor.
However, LEDs are point light sources, the emitted light diverges in a conical space, and the light intensity reduces with the increase of distance from the point light sources. If it is intended to cover the entire scanning area, the LEDs have to be arranged in a long array to emit light onto the entire scanning area. However, the area between two neighboring LEDs in the LED array only generates a weak light intensity depending on the scattering effect of the air. Therefore, if the LED array is directly used to emit the scanning light, the intensity of the light on the scanning area may be inconsistent. The solution for overcoming the problem of the inconsistent light intensity is not provided in the TW518877.
In order to solve the problem that the LEDs cannot provide the linear light source, the conventional method uses light guide elements such as prisms, cylindrical lenses, or transparent columns. The LEDs emit light onto two ends of a light guide element. Then, the light is uniformly projected from the lateral side of the light guide element because of the guiding and refracting light properties of the light guide element. For example, in R.O.C Taiwan patent TW549768 and I253840, a design utilizing the light guide element is provided, in which the LEDs become the linear light source. But, such a design of the light guide element has the disadvantage that the intensity of the light decays rapidly when the light travels in the light guide element, so the length of the light guide element is limited, resulting in that a width of the scanned document cannot be easily increased. In addition, a temperature due to heat generated during the operation of the LED causes degradation of the material of the light guide element, and thus the material used to be transparent becomes nontransparent or exhibits color. Therefore, the light guide element can not guide light anymore. Accordingly, the design adopting the light guide element cannot effectively solve the problem of using the LED to provide the scanning light source.